Field of Invention
The present invention relates generally to the field of cryostats. More specifically, the present invention is related to reducing noise and temperature during measurements in cryostats.
Discussion of Related Art
Since the 1960s, the most widely used type of cryostat for low-temperature measurements (i.e. below 1° Kelvin) is the dilution refrigerator. Presently, cryostats are commercially available from a variety of manufacturers (e.g., Oxford Instruments, Leiden Cryogenics, Janis Cryogenics, BlueFors, ICEoxford, and Cryoconcept) and they are used in most low-temperature laboratories world-wide. Typically, a dilution refrigerator is specified by the manufacturer to cool a device under test (DUT), such as semiconductors, crystals, complex oxides, novel electronic devices, quantum bits, etc. with a certain cooling power and base temperature. In this respect, temperature is understood as the phonon temperature (lattice vibrations) of the DUT. Due to the suppression of electron-phonon coupling at low temperatures and the sensitivity to electrical noise, the actual temperature of the electrons in the DUT is typically much higher than the phonon temperature, particularly if the DUT is characterized by electrical measurements. In fact, it is very challenging to obtain an electron temperature as low as the phonon temperature and, hence, commercially available cryostats do not specify their cooling performance with regards to electron temperature.
However, for the majority of customers, it is the electron temperature in the signal lines as well as the electrical noise environment that is most important; for example, for the performance of low temperature electronic devices, quantum bits, etc.
For this reason, many research groups have developed devices that can reduce the electron temperature and electrical noise. These devices are installed inside the cryostats to extend their cooling performance to the electronic domain. Some of these efforts have been going on for many years with varying degrees of success, and some techniques have been published. Examples of such published techniques/devices can be found in the paper to Lukashenko et al. entitled “Improved Powder Filters for Qubit Measurements”, the paper to Milliken et al. entitled “50Ω Characteristic Impedance Low-Pass Metal Powder Filters”, the paper to Santavicca et al. entitled “Impedance-Matched Low-Pass Stripline Filters”, the paper to Bluhm et al. entitled “Dissipative Cryogenic Filters with Zero DC Resistance” and the paper to Martinis et al. entitled “Experimental Tests for the Quantum Behavior of a Macroscopic Degree of Freedom: The Phase Difference Across a Josephson Junction”. These homebuilt devices are often unreliable, difficult to build, or large in size. One major vendor of dilution refrigerators (Leiden Cryogenics BV, The Netherlands) is offering cryostats with integrated devices to reduce the electron temperature and electrical noise on each individual electrical signal line.
Oxford Instruments, UK, intends to develop a comparable or better product for their cryostats. Since approximately 2005, the majority of cryostats sold by Oxford Instruments are so-called cryogenfree dilution refrigerators that employ a pulse tube cooler and cryostat-mounted turbomolecular pump, wherein such a setup allows continuous operation without expensive cryogens (such as liquid helium), but adds electrical noise induced by mechanical vibrations and magnetic induction. The resulting electrical noise can only be filtered inside the cryostat, and hence methods and devices that allow signal conditioning at low temperatures are needed even more than before the advent of cryogen-free dilution refrigerators.
Further, although a huge variety of commercial filters for electrical signal condition near room temperature (−55° C. . . . 100° C. Celsius), none of these are specified to work at cryogenic temperatures.
Embodiments of the present invention are an improvement over prior art systems and methods.